Kotlin Android Room error： Cannot figure out how to save this field into database

发布于05月25日

详细日志(log)

error: Cannot figure out how to save this field into database. You can 
consider adding a type converter for it.
private final java.util.Date mTime = null;

我有一个实体，其字段为

var mStartTime : Date = Date() // java.util.Date

Why cant Room persist Date objects? What can be best converter for Date?

For example, if we want to persist instances of Date, we can write the following TypeConverter to store the equivalent Unix timestamp in the database:
public class Converters {
    @TypeConverter
    public static Date fromTimestamp(Long value) {
        return value == null ? null : new Date(value);
    }
    @TypeConverter
    public static Long dateToTimestamp(Date date) {
        return date == null ? null : date.getTime();
    }
}
The preceding example defines 2 functions, one that converts a Date object to a Long object and another that performs the inverse conversion, from Long to Date. Since Room already knows how to persist Long objects, it can use this converter to persist values of type Date.

接下来，将@TypeConverters注释添加到AppDatabase类，以便Room可以使用您为该AppDatabase中的每个实体和DAO定义的转换器:

AppDatabase.java
@Database(entities = {User.class}, version = 1)
@TypeConverters({Converters.class})
public abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}

旁注:java.util.Date被认为设计糟糕(java.util.Calendar更糟).如果您有任何非平凡的日期时间逻辑，并且可以使用API级别26(桌面上的Java 8)，通常最好使用java.time package.如果不能，请参阅https://github.com/JakeWharton/ThreeTenABP了解后端口.